1. Field of the Invention
The present invention relates to a semiconductor storage device and a manufacturing method thereof, and more particularly to a semiconductor storage device having a high-dielectric-constant insulator and a manufacturing method thereof.
2. Description of the Related Art
With the miniaturization of a semiconductor storage device, not only a memory cell storing information has been miniaturized but also a space between adjacent memory cells. In a conventional storage device, especially in a non-volatile semiconductor storage device, an inter-electrode capacitance between a floating gate electrode and a control gate electrode is increased by forming an inter-electrode insulator not only on an upper surface of the floating gate electrode but also on a part of a side surface thereof. The floating gate electrode serves as an electric charge storage layer and the control gate electrode serves as a word line. When a space between the memory cells is reduced, there occurs a problem, e.g., an increase in a wiring line delay occurs due to capacitance coupling caused by a parasitic capacitance between side surfaces of adjacent memory cells.
As one means for solving this capacitance coupling problem, there is a structure which uses a flat memory cell having a flat inter-electrode insulator. Such a flat memory cell has advantages, such as a manufacturing process can be simplified and stabilized. On the other hand, as the inter-electrode insulator in the flat memory cell, it is necessary to use a high-dielectric-constant insulator having a relative dielectric constant higher than that of an ONO film (a three-layered film consisted of a silicon oxide (SiO2) film, a silicon nitride (Si3N4) film and a silicon oxide (SiO2) film), which is a conventional inter-electrode insulator.
Since the high-dielectric-constant insulator generally contains many grown in defects therein, it inherently has a problem that desired electrical characteristics can not be obtained in the grown high-dielectric-constant insulator. As example of the defects, for example, there are oxygen defects and the like due to an insufficient reaction or insufficient supply of oxygen when forming the high-dielectric-constant insulator and/or oxygen defects in the vicinity of a surface caused by processing in reducing atmosphere when depositing, e.g., an electrode material on the high-dielectric-constant insulator. Such defects act as electron trap sites, and hence a leak current of the high-dielectric-constant insulator is increased.
Jpn. Pat. Appln. KOKAI Publication No. 2001-185548 discloses a technology which compensates an oxygen defect in a tantalum oxide (Ta2O5) film which is one of high-dielectric-constant insulators. According to this technology, after forming the Ta2O5 film directly on a silicon substrate, an ultraviolet light is first applied to the Ta2O5 film in an atmosphere including nitrogen (N) and oxygen (O), e.g., nitric oxide (NO), heated to approximately 700° C. As a result, nitrogen radical and oxygen radical are generated. The nitrogen radical diffuses through the Ta2O5 film to reach the silicon substrate. Diffused nitrogen radical is coupled with a dangling bond of silicon at the silicon substrate surface to form SiON, whereby the silicon surface is stabilized. Then, a rapid heat treatment is carried out in an oxidizing atmosphere at approximately 800° C. to recover oxygen defects in the Ta2O5 film. Although oxygen diffuses to the silicon substrate surface during the heat treatment, since the silicon surface is stabilized by SiON, and hence silicon is not oxidized. Since the heat treatment must be carried in two stages and an ultraviolet light must be applied while heating, this technology has a problem, e.g., an increase in manufacturing steps, a manufacturing apparatus being complicated, and others.